1. Field of the Invention
The present invention relates to a casting technique. In particular, it relates to a technique for filling a cavity with melt (i.e. molten metal). A casting machine employing this invention is equipped with a holding furnace that stores the melt, a mold with a cavity formed in its interior, a melt duct that interconnects the holding furnace and the cavity, and a device that generates a pressure difference between the pressure inside the holding furnace and the pressure inside the cavity, and is characterized in that said cavity is filled by the melt inside said holding furnace by way of said melt duct due to this pressure difference.
2. Prior Art
Prior art that relates to this is disclosed in Japanese Laid-Open Patent Publication JP-A-59-10461, and FIG. 4 shows a schematic view of a casting machine that employs this method.
This low-pressure casting machine 1 is equipped with holding furnace 6 that stores the melt, and mold 3 positioned directly above this holding furnace 6 by fixing plate 2. Cavity 4 is formed in the interior of mold 3. A tubular melt duct 5 is connected to mouth piece 4h of said mold 3, and interconnects cavity 4 formed inside mold 3 with the interior of holding furnace 6. Here, said cavity 4 is released to atmospheric pressure via exhaust ducts (not illustrated), while on the other hand said holding furnace 6 is sealed and compressed air is supplied to the interior thereof by compressor 7. It is thus possible to generate a pressure difference between the pressure inside cavity 4 and the pressure inside holding furnace 6.
Said compressor 7 is made able to vary (increase) the pressure in said holding furnace 6 according to a prescribed pattern, and this variation of pressure causes the melt inside holding furnace 6 to be filled into cavity 4 through melt duct 5. Here, the difference in level between the surface of the melt filled in cavity 4 through said melt duct 5 and the surface of the melt inside holding furnace 6 is proportional to the pressure difference between the pressure inside the cavity and the pressure inside the holding furnace. It is thus possible to control the surface level of the melt filled into cavity 4 by controlling the pressure inside holding furnace 6. It is also possible to control the rate at which the melt rises by raising the pressure inside holding furnace 6 according to a prescribed pattern.
The pressure control method in this low-pressure casting machine 1 establishes a three-tier pressure pattern that is divided between the period during which the melt rises through melt duct 5 to the entrance of cavity 4, the period during which the melt is filled into cavity 4, and a feeder head pressurizing stage.
That is, in the stage during which the melt is supplied as far as the entrance of cavity 4, solenoid valves 8a and 8b of compressor 7 are opened and a large amount of compressed air flows into holding furnace 6 through pipelines 9a and 9b. Accordingly, the pressure inside holding furnace 6 rises quickly and the melt rises up at high speed inside melt duct 5 to arrive at the entrance of cavity 4. Next, when the pressure in said holding furnace reaches a first prescribed pressure, the melt surface is considered to have risen to the entrance of cavity 4 and solenoid valve 8b is closed. Consequently, compressed air is only supplied to holding furnace 6 through pipeline 9b, and the rate of pressure increase inside holding furnace 6 is relaxed by the drop in the compressed air supply rate. As a result, the melt is slowly filled into cavity 4. Then, when the pressure inside holding furnace 6 reaches a second prescribed pressure, cavity 4 is deemed to have been filled with melt, and solenoid valve 8c is opened. Consequently, compressed air is supplied to holding furnace 6 through pipelines 9b and 9c, and the pressure rises quickly again so that the melt inside cavity 4 is subjected to feeder head pressurizing.
As mentioned above, in low-pressure casting machine 1, the melt surface is considered to have arrived at the entrance of cavity 4 when the pressure inside holding furnace 6 has reached a first prescribed pressure, whereupon the pattern of pressure increase is changed into a relaxed pattern. That is, the increase in pressure per unit time is reduced when it has reached the first prescribed pressure. Also, cavity 4 is considered to have filled up with melt when it has reached a second prescribed pressure, whereupon the pattern of pressure increase is changed into a steep pattern. That is, the increase in pressure per unit time is increased when it has reached the second prescribed pressure.
However, the occurrence of phenomena such as back pressure in cavity 4 and variation in the melt surface level in holding furnace 6 arising from a variation in the amount of melt stored in holding furnace 6 can result in the melt surface not actually reaching the prescribed positions when the pressure in holding furnace 6 has reached the first or second prescribed pressure. Conversely, it is also possible that the actual melt surface will rise above the prescribed positions.
In such situations, if operations are continued according to the pattern of pressure increase set initially, it will become impossible to change the pattern of pressure increase at the point where the melt surface has actually reached the entrance of cavity 4 and at the point where cavity 4 has actually been filled with the melt. Therefore, this can give rise to defects whereby, for example, air is mixed in with the melt by filling cavity 4 at high speed when it should be filled slowly, or conversely whereby the melt temperature drops due to the melt surface being brought up slowly inside melt duct 5 when it should be brought up at high speed. Also, if the feeder head pressure after filling is insufficient, problems such as pipes in the moldings can Occur.